Secondary batteries are widely used as a power source for portable devices such as cellular phones, digital cameras, and laptop computers, and are becoming common as a power source for vehicles and household use in recent years. Especially, lithium ion secondary batteries, which have high energy density and are lightweight, have become an energy storage device essential for life.
A lithium ion secondary battery has a structure in which a positive electrode sheet and a negative electrode sheet as electrode elements are separated by a separator. Each of the positive electrode sheet and the negative electrode sheet includes a portion with an electrode coating, formed by applying a combined agent containing an active material, a binder, and a conductive aid, etc., onto a current collector, and a portion with no electrode coating, onto which a combined agent has not been applied to be connected to an electrode terminal. These are contained together with an electrolytic solution in an outer case and the outer case is sealed. A positive electrode terminal and a negative electrode terminal are electrically connected at one end to the portion with no electrode coating of the positive electrode sheet and the portion with no electrode coating of the negative electrode sheet, respectively, and the other ends of the positive electrode terminal and the negative electrode terminal are extracted to the outside of the outer case.
For the outer case, flexible films including an aluminum sheet or the like with a thickness of tens to hundreds microns are frequently employed as a means for weight saving for a battery. Although flexible films are excellently lightweight, it is disadvantageous that they have strength lower than metal can cases with a large thickness and are vulnerable to an impact from the outside, and in addition the outer case swells significantly when a gas is generated from the battery element or electrolytic solution contained in the container.
Accordingly, techniques to remove NH3 or H2 present in an electrolytic solution in a step of manufacturing an electrolytic solution have been proposed.
For example, Patent Literature 1 describes removal of a residual impurity (an unreacted material and a by-product) in a method for manufacturing an electrolytic solution containing LiPF6, and discloses that, in the case that the residual impurity is a volatile substance or a gas, the impurity is removed through flowing an inert gas or vacuum treatment.